Current rapid expansion in liquid crystal display (LCD) applications is largely due to improvements in display performance. High contrast, good color reproduction, and stable gray scale intensities are important attributes for electronic displays that employ liquid crystal technology. With respect to contrast, a primary constraint with liquid crystal displays is the propensity for light leakage in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically, optimum contrast is obtained only within a narrow viewing angle, centered about the normal incidence to the display, and falls off rapidly as the viewing angle is increased. In color displays, light leakage not only degrades the contrast, but also causes undesirable color or hue shifts, degrading color reproduction. In addition to black-state light leakage, viewing angle constraints for typical twisted nematic liquid crystal displays are exacerbated by a shift in the brightness-voltage response as a function of viewing angle, due to the inherent optical anisotropy of the liquid crystal material.
Thus, one of the major factors that determine the quality of LCD displays is the viewing angle characteristic, which describes the change in contrast ratio relative to different viewing angles. It is desirable that contrast be maintained over a wide range of viewing angles, a known shortcoming with liquid crystal display devices. One way to improve the viewing angle characteristic is to insert a compensator (also referred as compensation film, retardation film, or retarder) with proper optical properties between the polarizer and liquid crystal cell, such as disclosed in U.S. Pat. Nos. 5,583,679; 5,853,801; 5,619,352; 5,978,055; and 6,160,597. Compensation film according to U.S. Pat. Nos. 5,583,679 and 5,853,801, based on discotic liquid crystals which exhibit negative birefringence, is widely used. This film offers improved contrast over wider viewing angles; however, it suffers larger color shift for gray level images, compared to a compensator made of liquid crystalline materials having positive birefringence, according to Satoh et al. “Comparison of Nematic Hybrid and Discotic Hybrid Films as Viewing Angle Compensator for NW-TN-LCDs,” SID 2000 Digest, pp. 347-349, (2000). To achieve comparable contrast ratio while reducing color shift, one compensation film solution uses a pair of liquid crystal polymer films (LCP), having orthogonally crossed optical axes, disposed on each side of a liquid crystal cell, as discussed by Chen et al. “Wide Viewing Angle Photoaligned Plastic Films,” SID 99 Digest, pp. 98-101 (1999). This paper states that “since the second LPP/LCP retarder film is coated directly on top of the first LCP retarder film, the total thickness of the final wide-view retarder stack is only a few microns thin.” Although such a method provides a very compact optical component, it is difficult to manufacture a compensation film having two LCP layers whose optical axes are orthogonally oriented. This is a particular challenge where the film substrate is web-fed, such as in a continuous, roll-to-roll manufacturing process.
In processing liquid crystal compensation films, photo-alignment methods are recognized to have advantages over earlier rubbing alignment methods. Using photo-alignment, a thin alignment medium, typically linear photo-polymerization media (LPP) is applied to a substrate and is then irradiated, typically using UV light, to provide a directional alignment bias. There are a number of photo-alignment methods, based on different photoreaction processes. In general, a photo-alignment method may be one of three basic types:                (1) Isomerization, as disclosed in U.S. Pat. No. 4,974,941, is a reversible process using laser light irradiation in which a monomer or single molecule is aligned using cis-trans-isomerization effects;        (2) Photo-dimerization, as disclosed in U.S. Pat. No. 5,602,661 employs photo-induced orientation and dimerization of polymer side-chains, including cross-linking; and        (3) Photo-dissociation uses light to anisotropically alter an alignment medium such as polyamic acid or polyamide or copolymer comprised of amic acid and imide.        
In one promising photo-dimerization method, discotic liquid crystal structures within a liquid crystal polymer (LCP) layer are applied over an LPP layer to take the preferred alignment direction. Most solutions for photo-alignment using this method direct collimated polarized UV light, at an oblique angle, onto an alignment LPP substrate to align polymer molecules in a desired direction that provides a pretilt for a subsequently applied LCP layer containing liquid crystal structures. It has been found that, for suitable performance, only a fraction of molecules in the LPP alignment layer need to be photopolymerized. Typical LCP media include diacrylates and diepoxides and similar cross-linkable liquid crystalline materials. LCPs may have inherent positive optical anisotropy, such as with diacrylates, or negative anisotropy and weak biaxial properties, such as with discotic liquid crystal materials.
A number of different photo-alignment media and techniques have been used to provide the necessary pretilt for different types of liquid crystal display media. For a suitable class of LPP media, optical apparatus that provides irradiation for alignment must meet the following criteria:                1. Exposure levels of 10-15 mJ/cm2, nominal.        2. Narrow range of wavelengths. The exact range that is suitable for alignment irradiation depends on the material. UV-B (280-320 nm) is the preferred range for many types of alignment substrate. Some wavelengths are preferably rejected in order to minimize unwanted effects on alignment or undesirable temperature effects. Rejection of unwanted wavelengths is especially important for efficiency in a roll-to-roll manufacturing apparatus in which a web of substrate traveling at hundreds of feet per minute is processed. At such high speeds, the necessary increase in radiation at desirable wavelengths can easily bring with it an increase in undesirable radiation levels from other parts of the spectrum. For example, UV light is efficiently produced by a class of lamps that excite mercury or ion-doped mercury molecules. Such lamps typically generate UV-C (200-280 nm), UV-B (280-320 nm), UV-A (320-400 m), visible light, and infrared light. For an embodiment where UV-B is chosen as the preferred spectral range, it would be desirable to limit the irradiance and total exposure on the web from other parts of the spectrum.        3. Uniform exposure dosage. Exposure dose is expressed in terms of energy per unit area. It has been found that dosage levels, alternately termed exposure levels, can provide acceptable alignment results even where dosage varies by as much as +/−50% across the irradiated surface area in some applications. However, reasonable compensation for dosage uniformity helps to obtain uniform alignment results, minimizing intensity level variations between levels at the middle of a substrate and at substrate edges.        4. Uniform direction of polarization. It does not appear to be important that the applied alignment radiation be highly polarized. However, for a class of LPP materials, best results are achieved when the exposure radiation has a highly uniform direction of polarization. For maintaining a high standard of quality and uniform alignment, it is preferable to provide a consistent direction of polarization, varying no more than 1 degree over the full irradiated surface. Of course, the problem of maintaining this tolerance for directional uniformity of polarization is accentuated when irradiating a large scale surface.        5. Oblique incident angles for pretilt. Typically, some deviation from normal incidence to the media is required in order to provide the necessary pretilt angle to the LPP material. For most applications, a broad range of incident angles, such as over a 10-70 degree range, is permissible. We will refer to this illumination as illumination with predetermined inclination where it is understood that the inclination refers to the average angle of the multiplicity of incident angles rather then a single angle of illumination.        
There have been some conventional systems developed that generally meet most of requirements 1-5 above for irradiating alignment media on a small scale. However, it can be appreciated that these requirements become particularly difficult to meet as the irradiated surface area, or exposure zone, increases. Conventional solutions are as yet poorly suited to the demands for efficiently irradiating a web-fed substrate, where the substrate is moved past the irradiation device at production speeds and the web width exceeds 1 m.
In addition to the requirement for large scale photo-alignment processing, there is also a need to provide a film having composite LPP/LCP structures in which two alignment surfaces have been treated so that their respective optical axes are close to 90 degrees, that is, orthogonal, with respect to each other. Conventional approaches have not yet provided a suitable solution for achieving this with a web-fed media.
Among proposed prior art solutions for photo-alignment are a number of scanning solutions:                U.S. Pat. No. 5,889,571 discloses an irradiation device for scanning linearly polarized UV across a substrate to achieve alignment layer uniformity. U.S. Pat. No. 5,889,571 emphasizes the importance of oblique radiation. This solution is best suited to a substrate provided in sheet form rather than to a substrate continuously fed from a web.        U.S. Pat. No. 6,295,110 discloses a laser light-based system for applying polarized UV radiation across a substrate.        Designed for substrates having a diagonal in the range of about 10 inches or slightly larger, the U.S. Pat. No. 6,295,110 solution provides two-dimensional irradiation over an area that exceeds the size limit for the type of optical radiation employed. However, there are practical limitations in scaling this type of solution to suit a web-fed substrate having a width dimension of lm or larger.        
It has been noted that high irradiance conditions would be beneficial for use in high-speed roll-to-roll manufacturing apparatus, particularly where it is desirable to provide a compact system. Peak irradiance on the web in such environments could range from approximately 50 milliwatts/cm2 to values of several hundred milliwatts/cm2. This means that average irradiance on any polarizer would be much higher. With irradiance over ranges such as might be supplied using a medium pressure long-arc Mercury lamp at power levels in the 100-600 W range, conventional, resin-based polarizers would not be well-suited. For example, this type of irradiation exceeds the practical working range of polarizers such as HNP′B-Linear Polarizer from 3M (St. Paul, Minn.). Sheet polarizers are not generally capable of handling higher irradiation levels and may quickly deteriorate over a prolonged exposure period. With this limitation in mind, prior art solutions for providing polarized irradiation over a large area include the following:                U.S. Pat. No. 6,190,016 discloses an irradiation apparatus using an oval focusing mirror, integrator lens, and polarizer disposed at various points in the optical system. U.S. Pat. No. 6,190,016 emphasizes the value of collimated light, incident to a polarizer, to improve polarization performance. The use of Brewster plate polarizers for large scale surfaces is disclosed.        U.S. Pat. No. 5,934,780 discloses an exposure apparatus using a UV light source having an oval focusing mirror, where the apparatus includes an integrator lens, polarizer, and collimation optics. Brewster plate polarizers are used in the preferred embodiment. This type of solution may work well for a substrate up to a certain size. However, there are practical size limitations that constrain the use of Brewster plate polarizers for large substrates. Similarly, EP 1 020 739 A2 discloses a modified Brewster plate arrangement As a variation on Brewster plate polarizers, EP 1 172 684 discloses a modified V-shaped Brewster's angle arrangement. However, similar weight and size constraints also limit the feasibility of this type of solution.        
U.S. Pat. No. 6,292,296 discloses a large scale polarizer comprising a plurality of quartz segments disposed at Brewster's angle, used for system that irradiates using UV. However, such an arrangement would be very costly and bulky, particularly as a solution for a web-fed exposure system with a large irradiation area.
As the above-noted patent disclosures show, irradiation apparatus designed for large exposure zones have employed sizable polarization components, typically quartz or glass plates disposed at Brewster's angle. Hampered by the relative size and weight of these polarizers, such irradiation apparatus are necessarily less efficient in delivering light energy to the exposure surface. Moreover, conventional polarizers using Brewster plates or interference polarizers based on Brewster's angle principles also exhibit a high degree of angular dependency. That is, incident light must be substantially collimated in order to obtain a uniform polarized light output.
Significantly, Brewster plate polarizers such as those shown in the U.S. Pat. No. 5,934,780 and U.S. Pat. Nos. 6,061,138 and 6,307,609 are not optimal for providing a uniform polarization unless highly collimated light is used. With respect to an irradiated surface, the principal axis of polarization of the Brewster plate polarizer is uniform only when the plane of the Brewster plates is within a very limited range of angles. Otherwise, the Brewster plate polarizer does not have a well-defined, uniform principal axis of polarization. With Brewster plate polarizers, the direction of polarization is dependent upon the angular direction of incoming light. For each beam direction, a specific local coordinate system, aligned with the meridional plane containing incident and outgoing beams, is established at the point of incidence, as is shown in FIGS. 16a and 16b. Thus, when there are several incoming beams at different angles, the Brewster plate polarizer correspondingly provides multiple polarization directions, that is, multiple polarization axes. Moreover, the Brewster plate polarizer operates in one direction only; it would not be practical to use Brewster plate polarizers for achieving orthogonal polarization of multiple LPP alignment layers on a web-fed substrate. Instead, in order to provide orthogonal alignment of overlapping LPP layers, it would be necessary to expose individual cut sheets of media, rotating the media to obtain orthogonal exposure. Brewster plate solutions are not compact or practical for use with long-arc lamps, particularly where orthogonal exposure directions must be obtained.
Referring to FIGS. 16a and 16b, there is shown, for a Brewster plate polarizer 132, how a principal axis of polarization 126 varies with incident beams 124 at different angles. As is shown in these figures, a meridional plane 130 is defined by incident beam 124, a reflected beam 120, and a transmitted beam 122. Principal axis of polarization 126 has a variable angle ψ1 or ψ2 relative to a reference direction 128 depending on the incident angle of incident beam 124. Viewed geometrically, a tilt of meridional plane 130 results in a change to principal axis of polarization 126.
In contrast, conventional sheet polarizers have the property of providing a uniform principal axis of polarization for light from within a range of incident angles. Sheet polarizers are also capable of being rotated to allow orthogonal alignment exposure such as would be required in a continuous web-based manufacturing process. However, sheet polarizers are not robust under conditions of high UV light irradiance and would deteriorate rapidly. Thus, it can be seen that it is difficult to obtain efficient polarization of UV-B light (280-320 nm) at relatively high irradiance levels and for incident light at relatively wide angles of incidence using conventional polarization components and techniques.
Conventionally, wire grid polarizers have been used in infrared and longer-wavelength applications. More recently, wire grid polarizers have been developed for use with visible light, as disclosed in U.S. Pat. Nos. 6,234,634 and 6,243,199. Although the concept of wire grid polarizers for UV applications had been experimentally demonstrated in 1983 (see Sonek et al. “Optical polarizers for the ultraviolet” Appl Opt. 22, pp. 1270-1271; where evaporated aluminum was spaced at 115 nm on quartz substrate to cover a wavelength range of approximately 200-800 nm), only recently have wire grid polarization devices been commercially available for use with light in the UV range. Wire grid polarizers have inherent advantages in high-heat and high-irradiance applications where conventional sheet polarizers would not be suitable. Wire grid polarizers are also inherently less angularly dependent than other types of polarizers, particularly Brewster plate and interference type polarizers. Advantageously, wire grid polarizers have a low dimensional profile, allowing them to be used to replace sheet polarizers where space along the optical axis may be minimal. In addition, wire grid polarizers exhibit favorable response, similar to that available with sheet polarizers as noted above, with respect to principal axis of polarization. As is shown, for example, in FIGS. 16c and 16d, wire grid polarizers 134 provide a principal axis of polarization 126 that is fairly uniform with respect to a reference direction 128 when incident beams 124 have a range of incident angles. This capability means, for example, that wire grid polarizer 134 can be tilted with respect to incident beam 124 without a corresponding change in principal axis of polarization 126. In this way, principal axis of polarization 126 of wire grid polarizer 134 is independent of the angle of incidence of incident beams 124, over a broad range of angles. However, wire grid polarization devices are not dimensionally scaled to suit the requirements of applying polarized light over a large exposure zone.
Alternatively, the Beilby-layer polarizer, commercially available from Sterling Optics (Williamstown, Ky.), has desirable properties for efficiently polarizing light in the UV spectrum. This type of polarizer uses an azo-dye applied and fixed to a uni-directionally polished plate of fused silica. The subsequent angular acceptance angle for the Beilby-layer exceeds that of either commercially available interference filters, Brewster plates, or UV wire grid polarizers, and surface resilience to high heat or irradiance is superior to that of resin-based sheet polarizers. The Beilby-layer polarizer also exhibits a low dimensional profile and a favorable response with respect to principal axis of polarization.
A number of different types of light sources for photo-alignment have been disclosed, for example:                WO 00/46634 discloses a method for alignment of a substrate using an unpolarized or circularly polarized source, applied in an oblique direction.        U.S. Pat. No. 4,974,941 discloses alignment and realignment, preferably using a laser source.        U.S. Pat. No. 5,389,698 discloses use of linearly polarized UV for photopolymer irradiation. Similarly, U.S. Pat. No. 5,936,691 discloses use of linearly polarized UV for photopolymer irradiation, with the UV source positioned close to the substrate surface.        
As noted above, the use of collimated or substantially collimated light is, in large part, dictated by polarizer characteristics. In related exposure processing applications, collimated light is considered advantageous, as in these examples:                U.S. Pat. No. 5,604,615 and EP 0 684 500 A2 disclose forming an alignment layer by directing collimated UV through slits in a photomask.        In a related curing application, U.S. Pat. No. 6,210,644 discloses directing UV through slatted collimator for curing resin.        
U.S. Pat. Nos. 6,061,138 and 6,307,609 disclose a method and apparatus for alignment using exposure radiation that is “partially polarized” and “partially collimated.” By “partially polarized,” this disclosure identifies a broad range of S:P values from 1:100 to 100:1 with preferred range from 0.5:1 to 30:1. By “partially collimated” these disclosures identify a broad range with a divergence, in one direction, greater than about 5 degrees and less than about 30 degrees. The use of such broad ranges simply seems to indicate that some significant degree of variability is acceptable for both polarization and collimation. Indeed, in practice, most polarizers work within the broad range stated in U.S. Pat. No. 6,061,138, particularly over sizable exposure zones. As is generally well known and shown in the disclosure of U.S. Pat. No. 6,190,016, some degree of collimation is needed simply for consistent control of polarization. Partial collimation, over the broad ranges stated in U.S. Pat. No. 6,061,138 occurs when light simply passes through an aperture and is not otherwise blocked, focused, projected, or diffused. Baffles or apertures that block stray light necessarily perform “collimation” within the ranges given in the U.S. Pat. No. 6,061,138. Earlier work, disclosed in U.S. Pat. No. 5,934,780 similarly shows use of partially collimated light having relatively poor polarization and the use of relatively high incident angles for exposure energy, covering the ranges specified in the U.S. Pat. No. 6,061,138. Another earlier patent, EP 0 684 500 A2, states that collimation of the irradiating polarized light beam is preferable, but does not require collimation.
Thus, prior art seems to indicate that collimation, considered by itself, is not as important as other characteristics of exposure radiation. Certainly, some degree of collimation is inherently necessary in order to efficiently collect and direct light onto a substrate, taking advantage of the light emitted in all directions by using devices such as using reflective hoods, for example. As is noted above, some degree of collimation is necessary for polarizing light, since polarization devices are not typically equipped to handle wide variations in incident light divergence. But, taken in and of itself, collimation may have secondary importance relative to other properties of the exposure light.
In contrast, maintaining a consistent polarization direction or azimuthal angle appears to be very important for obtaining good results. The direction of polymerization or selection for LC alignment materials closely corresponds to the direction of polarization. In fact, there is evidence that partial polarization, as suggested by U.S. Pat. No. 6,061,138 and as exhibited in earlier work disclosed by Schadt et al. “Surface-Induced Parallel Alignment of Liquid Crystals by Linearly Polymerized Photopolymers” Japanese Journal of Applied Physics, Vol. 31, 1992, pp. 2155-2164, appears to be acceptable, provided that a consistent direction of polarization is maintained. The disclosure of U.S. Pat. No. 5,934,780 emphasizes the importance of this direction of polarization. It has been shown that optimal results are obtained over the exposure zone when the exposure energy is somewhat uniformly distributed and when the direction of polarization is tightly controlled to within about 1 degree.
As is shown in the prior art solutions cited above, achieving polarization over a broad exposure zone, with a tightly controlled direction of polarization, is particularly difficult with high intensity TV-B radiation. It is difficult to obtain a UV-B source that provides polarized UV-B light at reasonable cost. Moreover, high heat and irradiance requirements place considerable demands on filtering and polarization components. Conventional resin-based sheet polarizers are unlikely to withstand the elevated irradiance and high heat conditions. Brewster plates and interference filters can withstand these conditions but have size and weight disadvantages as well as acceptance angle constraints.
As a further complication, controlling the intensity of radiation energy has been proven to be difficult to achieve and maintain on a substrate. Obtaining a uniform dosage distribution from a light source requires a means for redirecting the energy applied so that the intensity at the edges of the illuminated surface is not appreciably different from that at the center of the surface.
While tolerances may not be critical, some reasonable degree of uniformity appears to be desirable. Dosage is the product of irradiance and exposure time. If the substrate to be exposed is on a moving web, the dosage is an integration of the dosage received along the direction of motion over the time of exposure. The maximum permissible dosage variation over the substrate depends on the resultant properties desired over specific spatial distances. Differences in dosage applied to an LPP layer will subsequently influence alignment of the adjacent LCP layer. Reasonable compensation for dosage uniformity helps to obtain uniform alignment results. Dosage uniformity may be slowly varying across large spatial distances; for example, as would be typical for a single lamp located above and near the center of a substrate, dosage would be at a maximum near the middle of the substrate and decrease monotonically towards the outer substrate edges. Alternatively, other equipment designs that include other reflecting, absorbing, refracting and diffracting structures disposed between the lamp and substrate may impart changes in dosage that vary non-monotonically from the center to edge of a substrate; variations in dosage may vary over small patches or over small strips. These variation may be even more important to control than the slowly varying change because the slope of the change may be large.
In applications where light throughput is important, standard diffusers are inefficient at correcting the problem of dosage uniformity because of the wide dispersal of the incident radiation and diminished throughput relative to polished optical surfaces. A standard diffuser is typically a piece of transparent material ground, etched or molded on at least one side with small features that refract and scatter light into a hemisphere. A significant fraction of the light incident on a standard diffuser is typically reflected and scattered back uselessly. Additionally, the wide dispersal of incident radiation into a hemisphere is at odds with the goal of maintaining an average angle of inclination for aligning an LPP layer.
Alternatively, there are other methods that partially overcome some of the aforementioned difficulties of maintaining the average angle of inclination of the incident light and reducing the fraction of light that is uselessly reflected and back-scattered. One method is to use a “fly's eye” optical integrator, such as those employed in solar simulators like those from Oriel Corporation (Stratford, Conn.). Also, the long-known method of using an integrating bar for mixing light can be applied. For example, U.S. Pat. No. 1,917,360 discloses the use of an integrating bar with diffuser for a film printer. More recent applications of integrating bars, as exemplified by WO 01/80555 A1, do not use diffusers on the integrating bar and consequently maintain better directionality of the light cone. Light homogenizers like “fly's eye” and integrating bars are typically used in projection systems where the size of the device is not the foremost consideration; systems with an integrating bar or “fly's eye” lens typically scale with the maximum dimension of the substrate to be irradiated. Consequently, the approach of using a single projection device employing a “fly's eye” optical integrator or integrating bar is less amenable to being adapted to irradiate and align increasingly large substrates. Using a single small instrument that scans a large substrate raises problems with the rapidity of alignment. Using multiply operated projection instruments to expose a substrate in separate swaths or small patches increases the logistical problems of aligning and managing multiple projection units to achieve consistent uniformity of exposure.
Cost-effective mass manufacture of LC material requires high throughput. This necessitates using sufficient intensity levels, consistently applied to a material that is exposed and cured at fast speeds. Although conventional solutions provide some capability for high-volume web-fed manufacture, there is clearly room for improvement in processing speed, cost, and quality over prior art approaches.
It can be appreciated that there would be benefits to manufacturing apparatus and methods for fabrication of a compensation film having orthogonally disposed optical properties. Such a film would enhance the viewing angle performance liquid crystal displays, for display technologies including twisted nematic (TN), super twisted nematic (STN), optically compensated bend (OCB), in plane switching (IPS), and vertically aligned (VA) liquid crystal displays. These various liquid crystal display technologies are described in U.S. Pat. Nos. 5,619,352; 5,410,422; and 4,701,028. Conventional approaches do not provide a suitable solution for mass-manufacture fabrication of such a compensation film.
Thus, it can be seen that there is a need for improved apparatus and methods for fabricating a liquid crystal display compensation film provided as a web-fed substrate, where the film comprises two orthogonally oriented alignment layers.